While some Koreans discuss blood types to break the ice, many unconsciously or intentionally use it to gauge personality traits associated with each type – a practice that has invited discrimination against some.

A Gallup Korea study published in October showed up to six out of 10 adult Koreans believed blood types indicated certain personality traits, also referred to as the blood type personality theory.

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The study, which surveyed 1,500 men and women over 19 years of age, showed 58 percent believed blood type corresponded to personality traits, with 49 percent of them saying they liked people with type O blood the most. The next most popular were type A (20 percent), type B (16 percent), and type AB (6 percent).

Type O blood had also ranked number one in the same survey conducted in 2012 and 2002, Gallup Korea said. Many believe people with type O blood have an extroverted personality suited to collectivistic Korean culture and few character flaws, which may have arisen from the fact that type O blood is considered the “universal donor.”

Whether Koreans believe in the theory or not, all are, at the least, aware of what social image each blood type portrays.

People with type A blood are said to be introverted, indecisive, and timid; type Os are tolerant of others, well-rounded, and stubborn; type Bs are self-centered, moody, and extroverted; Type ABs are thrill-seeking, eccentric, and individualistic.

The pseudoscience began with Ludwik Hirszfeld, the Polish microbiologist, and serologist who labeled blood types as we know them today. In the early 20th century, Hirszfeld first theorized that blood groups indicated inferior or superior racial characteristics, with type A being the blood of the “superior” West and type B indicating inferiority. According to Spiegel Online, other scientists of his time, such as bacteriologist Max Gundel, believed type B blood was common among "psychopaths, hysterics, and alcoholics.”

Although Hirszfeld himself said at the end of World War II that blood group research “served a bad cause,” the pseudoscience was picked up by Takeji Furukawa, a Japanese professor at Tokyo Women’s Teacher’s School in 1926, who subsequently propagated the blood type personality theory.

Crossing over to Korea, the four blood types have become heavily associated with personality, most prominently in 2004 with a survey by MediTV indicating 75 percent believed the theory.

In 2004, a Nonghyup division drew public ire for saying that it will only hire people with type O and B blood types while asking people with type A and AB blood to refrain from applying since they “had no drive.”

News outlets also reported on blood type personality surveys. Local reports in 2013 showed 63 percent of women and 48 percent of men believed that they did not get along with certain blood types when dating. Type B blood had the worst reputation with 32 percent of women disliking men with type B blood, followed by type A (30.1 percent) type AB (12.5 percent), and type O (6.4 percent). Only 18.5 percent said they did not dislike a blood type.

Another outlet reported a 2010 survey from Job Korea that found 45.3 percent of employees in the finance and accounting industry had type A blood, 46.7 percent in the service industry had type B blood, 45.7 percent in sales had type O, and 37 percent in a specialized field had type AB.

Accumulation of discrimination cases and individual complaints pushed media to withdraw television shows that perpetuated the blood type personality theory since 2008, with some shows and documentaries actively disproving the theory.

While blood type personality theory is pseudoscience, other research has discovered that blood type does, in fact, correlate to certain physical immunities.

Research from Harvard School of Public Health found that blood types AB and B are at the highest risk for heart disease. The Journal of the National Cancer Institute says type Os are 37 percent less likely to develop pancreatic cancer than others.

A 2016 study from Seoul National University Bundang Hospital also showed that people with type B and AB blood have a lower risk of developing stomach cancer.

Meanwhile, type A blood was the most prominent in Korea, accounting for roughly 34 percent of the population, followed by type O (28 percent), type B (27 percent), and type AB (11 percent), according to Gallup Korea.